1. Field of the Invention
The present invention relates to a method for crystallizing an amorphous film, and more particularly, to a method for crystallizing an amorphous film by an electric field and an ultraviolet (UV) ray, and a method for fabricating a liquid crystal display device (LCD) by using the same.
2. Background of the Related Art
As devices become larger and more integrated, switching devices become thinner. As a consequence the present amorphous silicon thin film transistors are replaced with polycrystalline thin film transistors.
With a process temperature below 350° C., though the amorphous silicon thin film transistor can be fabricated on a glass substrate with ease, it is difficult to employ the amorphous silicon thin film transistor in a fast operation circuit due to low mobility.
However, since polycrystalline silicon has a mobility higher than amorphous silicon, a driving circuit can be fabricated on a glass substrate. Therefore, polycrystalline silicon is favorable for a switching device of a high resolution, large sized device.
Polycrystalline silicon may be formed by direct deposition of the polycrystalline silicon, or crystallizing amorphous silicon after the amorphous silicon is deposited. The latter method may include Solid Phase Crystallization (SPC), Excimer Laser Annealing (ELA), Metal Induced Crystallization (MIC), and the like.
A high crystallization temperature and a long heat treatment time period are essential in the SPC method, though SPC is comparatively simple since it only requires a lengthy time period of heat treatment in a furnace in which the temperature is more than 600° C. for forming the polycrystalline film. SPC has disadvantages in that fabrication of a device by SPC has many difficulties because SPC causes many defects inside the crystallized grains, and a glass substrate cannot be used due to the high crystallizing temperature.
The ELA method, in which an excimer laser with a short wave length and a high energy is irradiated momentarily for crystallizing a thin film, facilitates a low temperature crystallization at a temperature below 400° C., and produces a large sized grain with excellent properties. However, since ELA progresses non-uniform crystallization and requires expensive equipment, ELA is not suitable for mass production and fabrication of large sized devices.
The MIC method, introduced through research in decreasing the crystallization temperature, facilitates crystallization at a temperature below 500° C., and is favorable for fabrication of a large sized LCD.
Field Enhanced-Metal Induced Crystallization (FE-MIC) has been developed as an advanced form of MIC for crystallizing at a low temperature by using catalytic metal in an electric field. In the FE-MIC method, the crystallization temperature of a thin film decreases significantly when a metal impurity is added to an amorphous silicon film because free electrons of the metal decrease a bonding energy of the silicon due to an action of the free electrons of the metal.
FE-MIC is favorable for large sized glass substrate applications because crystallization time is shortened and the crystallization temperature decreases significantly compared to the present MIC method when an electric field is applied to the amorphous silicon film having the catalytic metal contained therein. In general, FE-MIC is influenced by an amount of catalytic metal; the more the catalytic metal, the lower the crystallization temperature.
The steps of a related art method for crystallizing an amorphous film, and the steps of a related art method for fabricating an LCD by using the same will be explained, with reference to the attached drawings.
First, the steps of a related art method for crystallizing an amorphous film will be explained.
Referring to FIG. 1A, a buffer layer 2 is formed on a substrate 1, and an amorphous silicon is deposited thereon at 300–400° C. by Plasma Enhanced CVD (PECVD), Low-Pressure CVD (LPCVD) using silane gas or by sputtering to form an amorphous silicon thin film 3. The buffer layer 2 prevents impurities in the substrate 1 from diffusing into the amorphous silicon thin film 3, and cuts off thermal flow to the substrate I in a later crystallization.
Next, referring to FIG. 1B, a metal, such as nickel, is deposited on the amorphous thin film 3 by using plasma to form a catalytic metal layer 4, and annealed by using a high temperature lamp. In this instance, the nickel atoms diffuse into the amorphous silicon thin film to form nickel silicide that accelerates the crystallization.
Then, referring to FIG. 1C, an electric field is applied to the amorphous silicon thin film 3 having the catalytic metal layer 4 formed thereon by means of electrodes 5 provided at both ends thereof. This causes growth of needle-like forms of crystalline grains in an <111> orientation by movement of the nickel silicide, leading to a decrease in bonding energy of the amorphous silicon thin film by the free electrons of the nickel atoms, and the crystallization time period is shortened as the nickel atom acts as a seed of the crystallization.
Thus, the amorphous silicon thin film 3 is crystallized into a polycrystalline silicon thin film. FE-MIC, a low temperature crystallization method which uses a catalytic metal in an electric field, is advantageous in that a crystallization rate is high, cost is low, and application to large sized glass substrates is possible.
A related art method for fabricating an LCD by using FE-MIC will be explained.
First, a buffer layer is formed of a silicon oxide on a thin film array substrate, and an amorphous silicon thin film is formed thereon. An electric field is applied to the amorphous silicon thin film while heating the amorphous silicon thin film, to crystallize the amorphous silicon thin film into a polycrystalline silicon thin film.
Next, the polycrystalline silicon thin film is patterned, to form an active semiconductor layer. Silicon nitride SiNx is deposited on an entire surface inclusive of the semiconductor layer, to form a gate insulating film.
Then, a low resistance metal film is deposited on the gate insulating film, patterned by photolithography, to form a gateline and a gate electrode, and impurities are injected into the semiconductor layer with the gate patterns used as mask, to form source/drain regions.
Next, source/drain electrodes are formed for crossing the dataline perpendicular to the gateline and the source/drain regions. The data patterns are insulated from the gate patterns by an interlayer insulating film.
Then, a protection film is formed on an entire surface including the source/drain electrodes, and a pixel electrode is formed connected to the drain electrode through the protection film, thereby completing fabrication of an array substrate of an LCD.
When a color filter substrate with a color filter layer and a common electrode is bonded to the thin film array substrate, and a liquid crystal layer is formed between the two substrates, the LCD is formed.
However, the related art method for crystallizing an amorphous film, and a method for fabricating an LCD by using the same have the following problems.
Application of the method for crystallizing an amorphous film to large sized glass substrates requires a low crystallization temperature and a uniform heat treatment throughout the substrate.
However, the related art heat treatment using a lamp is limited in applications to large sized devices due to difficulty in maintaining the uniformity of the heat treatment temperature which causes a problem of substrate deformation resulting from temperature rise of the large sized glass substrate.